1. Field of the Invention
This invention relates to a circuit for driving an electroluminescent (EL) display device.
2. Description of Related Art
A device disclosed in Japanese Patent Application Laid-Open Publication No. Hei 5-333815 is known as a circuit for driving an EL display.
According to this device, EL elements in the display are arranged in a matrix and a scan side driver IC and data side driver IC are respectively provided on a scan side and data side of the display elements. Accordingly, drive voltage pulses having a differing polarity with each positive and negative field are applied to the EL elements by the respective driver ICs and the EL elements emit light.
That is to say, in the positive field, a ground voltage (0 V) is taken to be a reference voltage and a voltage Vr corresponding to an EL drive voltage is output to a scan electrode of the EL display from the scan side driver IC, and from the data side driver IC, the ground voltage is output to the EL element so that it emits light, a modulation voltage Vm is output to a data electrode of an EL element to put it in a non-electroluminescent state, voltage of the data electrode is grounded with respect to the scan electrode to which the voltage Vr has been output, the Vr voltage is applied to the EL element, and the EL element emits light.
Additionally, in the negative field, ground voltage (0 V) is taken to be a reference voltage and a voltage of -Vr+Vm is output to the scan electrode from the scan side driver IC, and from the data side driver IC, the modulation voltage Vm is output to the EL element so that it emits light, the ground voltage is output to a data electrode of an EL element to put it in a non-electroluminescent state, voltage of the data electrode is set at the modulation voltage Vm with respect to the scan electrode to which voltage of -Vr+Vm has been output, -Vr voltage is applied to the EL element, and the EL element emits light.
In a case wherein drive such as the foregoing is performed, the power source voltages of the scan side driver IC becomes Vr and ground voltage when driving in the positive field, and so the voltage Vr is applied to the scan side driver IC, and consequently the breakdown voltage thereof must be Vr or higher. Because the EL element is driven at a comparatively high voltage which becomes, for example, approximately 260 V, a device with high breakdown voltage as the scan side driver IC becomes necessary. Because a general purpose driver IC does not have such a high breakdown voltage, it is necessary to specially design a high breakdown voltage driver IC which has a high breakdown voltage to satisfy the above requirements, and this causes problems in terms of integration and cost. These considerations also apply to the data side driver side IC.
Further, when the rear electrode has been grounded and a positive and negative alternating current signal is applied to the transparent electrode, two types of positive and negative power sources normally become necessary, and power source circuitry becomes large.
Also, driving the above-described prior art device, electrical charging and discharging with respect to the EL display panel is performed at each scan line electroluminescence operation, and there exists a problem wherein drive power consumption per cycle becomes large.
The device disclosed in Japanese Patent Publication Laid-Open No. 63-168998 attempts to solve this problem. After an EL element has emitted light, accumulated charge is stored in a capacitor provided externally, and this accumulated charge is reused during subsequent electroluminescence, thereby reducing power consumption.
As shown in FIG. 15, this circuit includes a data voltage supply circuit 7 having a charge collection capacitor 701, switching elements 702 through 704, and diodes 705 and 706, and Vm/2 is utilized as the power source voltage. The size of the charge collection capacitor 701 is sufficiently large in comparison with the charge capacity of the entirety of the EL display panel, and a charge equivalent to Vm/2 is charged therein as an initial state.
Operation of this device will be described hereinafter with reference to the graphs shown in FIGS. 16A-16F.
In the second field, when performing drive for a predetermined scan line, firstly the switching element 704 is switched on as shown in FIG. 16C, a P-channel FET connected to a data electrode of an EL element to emit light is switched on and a corresponding N-channel FET is switched off as shown in FIGS. 16D and 16E, and voltage Vm/2 is applied to the data electrode as shown in FIG. 16F.
Next, the switching element 702 is switched on as shown in FIG. 16A, voltage Vm being power source voltage Vm/2 with Vm/2 corresponding to a capacitor charge added thereto is applied to the data electrode of the EL element to emit light, and the EL element emits light. Subsequent to this electroluminescence operation, the switching elements 702 and 704 are switched off, and approximately half of the charge output from the P-channel FET of the data side driver IC 4 is collected via the diode 705 in the charge collection capacitor 701.
The collected charge is consumed when switching on the switching element 704 in the subsequent scan line selection period. This operation is repeated until the final line.
However, structure and operation of the charge collection capacitor 701 of the above-described device are complex, and there are problems where applied voltage at the time of the next scan line selection is affected by an amount of accumulated capacitor charge and is unstable, and so on.